One-way or overrunning clutches transmit torque in one direction only and are generally used to disengage a drive connection automatically when torque is applied in the reverse direction. Overrunning clutches commonly comprise two members having cylindrical, concentric, opposed faces defining an annular space therebetween. The inner face is keyed or otherwise fixed to an input shaft, and the outer face is fixed to a housing, or hub or sleeve, from which the drive is ultimately taken. Alternatively, the outer face may form the torque input member and the inner face may form the torque output member.
Generally, overrunning clutches use a ratchet mechanism, sprag elements, or roller elements to effect the transmission of torque in only one direction.
In a ratchet-type of overrunning clutch, a series of ratchet teeth is, for example, attached to the input shaft and a pawl is attached to the output housing. The pawl is spring-loaded and the teeth are oriented so that when the input shaft is rotated in one direction, the pawl engages with the teeth to transmit torque from the input shaft to the output housing. When the direction of rotation is reversed, the pawl is released form the teeth and no torque is transmitted.
In a sprag-type of overrunning clutch, a series of rocking tumblers or sprags is held together within a cage. The cage typically includes two rings and a crimped ribbon spring in the annular space between the rings. All three components of the cage are pierced to accommodate the sprags. The ribbon spring biases the sprags upright between the inner and outer races of the drive coupling. Because the radial clearance between the races is insufficient for the sprags to rock all the way to their top dead center positions, the sprags jam between the races. As long as the drive is in the direction tending to keep the sprags thus jammed, torque is transmitted from the input shaft, through the inner race, and finally to the outer race. Whenever the drive is reversed, the sprags tend to lay down against the influence of the ribbon spring, and no torque is transmitted.
Similarly, a roller-type of overrunning clutch uses rollers housed in inclined recesses formed in the annular space between the inner and outer races. When the shaft is rotated in one direction, the rollers run up the inclines and jam between the inner and outer races. If the shaft is thereafter rotated in the other direction, the rollers run down the inclines and are freed in the space between the two races, which is wider than the diameter of the rollers. To obviate backlash in the system, the rollers may be spring-loaded up to the inclines.
Each of the described overrunning clutch types have disadvantages when employed in certain high performance applications such as in the clutch assemblies of racing or mountain bicycles. For example, when a ratchet clutch is rotated in a forward direction, the pawl runs along the teeth resulting in a "clicking" noise. Furthermore, when the direction of rotation is reversed, a ratchet clutch generally requires 9 to 12 degrees of rotation before the housing and shaft become sufficiently engaged via the ratchet and pawl to transmit torque. Unlike the ratchet overrunning clutches, sprag and roller clutches are quiet and engage instantly to transmit torque. However, sprag and roller clutches are used less frequently than ratchet clutches because the former generate large radial forces. These forces have an expanding effect on the outer housing used to contain the rollers or sprags and a contracting effect on the input shaft. To contain these stresses, housings and shafts for these clutches must be made much larger and heavier than housings and shafts for ratchet clutches. This is a great disadvantage in the bicycle market, for example, where bicycles are valued in large part by their lightweight, compact construction.
For the foregoing reasons, there is a need for an overrunning clutch that is both quiet and quick to engage, yet which is also of lightweight and compact construction.